1. Technical Field
The present invention relates to a semiconductor relay system using a semiconductor relay, and more particularly to a method for cutting off a current flow flowing to a load by a way of a microcomputer which surely detects an overcurrent when the overcurrent flows through the load due to short-circuit, and a circuit cutting-off device under the overcurrent.
2. Related Art
Generally, in a vehicle, power from an on-board battery is supplied to loads arranged at individual portions of the vehicle through a power MOSFET and a power source line covered with an insulating coating. The power source line is arranged along a vehicle body in an engine room which is vibrating all the time. In this case, if the power source line is located in the vicinity of the corner of a car body, its intermittent contact with the corner will be repeated by e.g. vibration. Further, if it continues for a long time, the coating of the power source line will be gradually cut away by the corner, and hence the inner conductive line may be exposed slightly. In such a case, the exposed portion of the power source line will be brought into contact with the car body. This leads to dead shorting or rare shorting in the power source line so that an overcurrent flows.
In recent years, a power MOSFET or IPS (Intelligent Power Switch) has been used as a semiconductor relay for a motor vehicle. In order to prevent heat generation of a device, a current limiter system as shown in FIG. 7 has been adopted in the semiconductor relay or IPS. The constant current system is a method of controlling a load current in fixed level by cutting down a gate voltage of FET under a voltage of steady state. Shutdown system is a method cutting the load current immediately after flowing overcurrent. Pulse width constant system is a method of cutting the load current intermittent while the overcurrent is flowed. PWM (Pulse Width Modulation) is a method of controlling a current level and a current applying time. For example, where an inrush current, which represents a large current flowing through a lamp load when a lamp switch is turned on, is taken in consideration, a PWM (Pulse Width Modulation) system in which the power MOSFET is completely turned off at a prescribed overcurrent is optimum. However, as shown in FIG. 8, when an overcurrent flows because of any cause (e.g. ground fault) while a current flows through a lamp load (a stationary current is supplied), the PWM system lowers the current flowing through the lamp load to zero. Therefore, to sample the current value at regular intervals using a microcomputer to read a current monitored value, since the sampled value may fall on the case where the current flowing through the lamp load is zero, like timing t2 in FIG. 8, the overcurrent could not be correctly monitored in software.
If there is an incomplete short-circuiting A of FIG. 8 in which a current not reaching a reference value for deciding the overcurrent in software flows, the overcurrent cannot be decided in software so that thermal stress is applied to the device (in the case of an normal FET). In the case of an incomplete short-circuiting B of FIG. 8, even when the decision in hardware cannot be made, the thermal stress is given to the device for a long time. Therefore, a thermal cutoff circuit incorporating type MOSFET (hereinafter referred to as a thermal FET) is also difficult to lengthen its life.
The conventional overheat cutoff function or overcurrent protection function incorporating the IPS is only the function of the self-protection, but does not consider protecting the wire harness or circuit arranged in a vehicle. Therefore, the conventional semiconductor relay or IPS has a disadvantage that it can execute the overheat cutoff in order to prevent heat generation, but cannot smoothly cut off the circuit under the overcurrent.
In above relay system, it was proposed to used a switching element of a thermal MOSFET that incorporates a thermal self-interrupting circuit and is cut off irrespectively of the voltage applied to a gate by its self-cutoff function. In this system, where a current larger than a prescribed current flows because of any cause (e.g. ground fault) while a current flows through the lamp load (i.e. a stationary current is supplied, generally), it is detected that the overcurrent flows through the lamp load at a prescribed time, thereby cutting off the thermal MOSFET. However, where it is not detected that the overcurrent has flowed through the lamp load, when the temperature of the thermal MOSFET exceeds its own thermal cutoff temperature as a result that the current has continued to flow through the thermal MOSFET, the thermal MOSFET is self-cutoff, thereby stopping the current supply to the lamp load.
Thus, the state where the current is not supplied to the lamp load includes two cases of: cutoff of the thermal MOSFET based on the general overcurrent control and cutoff of the thermal MOSFET based on the self-cutoff due to the heat generation of the thermal MOFET. Both these two cases are directed to stopping of the current supply to the lamp load by cutoff of the thermal MOSFET.
On the other hand, the thermal MOSFET has a life, and is particularly sensitive to heat stress. If it suffers the heat stress exceeding its own thermal cutoff circuit incorporating temperature and repeats the self-cutoff, the thermal MOSFET will be broken. Namely, the thermal MOSFET has the life attributed to the number of times of self-cutoff that is repeated whenever it suffers the heat stress to exceed the heat cutoff temperature. If the number of times exceeds that corresponding to the life, the thermal MOSFET will be broken.
Particularly, where the self-cutoff is repeated a number of times in an atmosphere at a low temperature (below the freezing point), it takes a time for the temperature of the thermal MOSFET to rise from the low temperature (below the freezing point) to the thermal cutoff temperature. In this case, the thermal MOSFET suffers more heat stress than when its temperature rises from the normal temperature to the thermal cutoff temperature. Therefore, the limitation of number of times becomes stricter to the repeat of the self-cutoff of the thermal MOSFET in the atmosphere at the low temperature (below the freezing point) than that in the normal temperature.
Where the self-cutoff is repeated a number of times in the atmosphere at a low temperature, particularly the heat generated in a chip is conducted to a stem thereof, the thermal MOSFET suffers heat stress so that it will be broken (e.g. number of times of endurance). The breakage may be attributable to xe2x80x9cAl spikexe2x80x9d and xe2x80x9cAl slidexe2x80x9d. The Al spike refers to the fact that when the device instantaneously reaches a high temperature and repeatedly exceeds Tch (four channel temperature), the Al junction portion is fatigued and the resistance increases so that heat generation is accelerated. Hence the Al electrode layer will be molten and diffused into silicon, and eventually the device will break down. The Al slide refers to the fact that owing to the difference between a package material and a chip material in their heat expansion, the Al wiring at a chip corner slides and eventually the device will break down.
When the thermal MOSFET breaks down, the reliability of the entire semiconductor relay system which is used as a switching device is deteriorated. Therefore, it is necessary to count the number of times the self-cutoff made as a result that the thermal cutoff circuit incorporating MIDFET has suffered the heat stress, and carry out the processing such as issuing warning before the thermal MOSFET breaks down.
However, as described above, the cutoff of the thermal MOSFET occurs two cases of that based on the general overcurrent control and that based on the self-cutoff due to the heat generation of the thermal MOFET. Conventionally, both in these two cases cannot be distinguished from each other so that the number of times of the self-cutoff made as a result that the thermal MOSFET has suffered heat stress cannot be counted.
An object of the present invention is to enable the overcurrent due to the short-circuiting on the side of a load to be sufficiently decided, and where a thermal cutoff circuit incorporating type MOSFET is used, to enable the overcurrent to be decided by the overheat cutoff function of the MOSFET itself even when the power loss at the time of control is great.
An object of the present invention is to enable the number of times of self-cutoff due to heat generation of a thermal MOSFET to be counted accurately.
In order to attain the above object, a method of cutting off a circuit under an overcurrent in a semiconductor relay system for supplying a power source to a load, the method includes the steps of:
setting a first prescribed value which is a current value higher than a rated load current for the load and is a maximum current value supplied to the load;
setting a second prescribed value which is a current value between the first prescribed value and the rated current value;
sampling a current value flowing to the lamp load at predetermined intervals;
comparing the sampled current value with the second prescribed value;
controlling the current supplied to the load within a range between the first prescribed value and the second prescribed value in case of the sampled current higher than the first prescribed value flows to the load; and
deciding the sampled current as an overcurrent and cutting off the semiconductor relay to stop the current supply to the load in case of the sampled current value higher than the second prescribed value is counted by a predetermined number of times.
In this configuration, the invention permits sufficient decision of an overcurrent due to short-circuiting on the side of the load.
In order to attain the above object, in the method of cutting off a circuit, the second prescribed value is several times as large as the rated load current.
In this configuration, when the overcurrent flows to the side of the load owing the short-circuiting, it can be surely detected.
In order to attain the above object, a circuit cutoff device having a semiconductor relay system for supplying a power source to a load under on/off control by a semiconductor relay of a microcomputer, the circuit cutoff device includes:
a first setting device setting a first prescribed value which is a current value as maximum current value supplied to the load, having a current detection resistor connected to a drain side of the semiconductor relay for detecting a current supplied to the load;
a comparator comparing a resistance voltage at both ends of the current detector resistor to a first prescribed voltage defined by the first prescribed value;
a hysteresis circuit applying a hysteresis having a prescribed width defined by the first prescribed voltage and a second prescribed voltage lower than the first prescribed voltage to the resistance voltage;
a gate logic circuit controlling a gate voltage of the semiconductor relay for stopping supplying the power source to the load according to an output from the comparator in case of the current exceed the first prescribed value, and controlling the gate voltage for supplying the power source to the load in case of the current lower than the second prescribed voltage; and
a second setting device setting a second prescribed value which is a current value lower than the first prescribed value and higher than a rated current supplied to the load in a stationary state.
In this configuration, in accordance with the invention, the overcurrent due to the short-circuiting on the side of a load can be sufficiently decided. In addition, even when the power loss at the time of control is great, the overcurrent can be detected using the overheat cutoff function of a thermal cutoff circuit incorporating type MOSFET itself.
In order to attain the above object, in the circuit cutoff device under an overcurrent, the semiconductor relay is a thermal cutoff circuit incorporating MOSFET.
In this configuration, in accordance with the invention, the overcurrent due to the short-circuiting on the side of a load can be sufficiently decided. In addition, even when the power loss at the time of control is great, the overcurrent can be detected using the overheat cutoff function of a thermal cutoff circuit incorporating type MOSFET itself.
In order to attain the above object, in the circuit cutoff device under an overcurrent, the second prescribed value is set at a current value which is several times as large as a rated load current.
In this configuration, in accordance with the invention, when the overcurrent flows to the side of the load owing the short-circuiting, it can be surely detected.
In order to attain the above object, the circuit cutoff device under an overcurrent, wherein the second prescribed value is set by software in the microcomputer.
In this configuration, in accordance with the invention, when the overcurrent flows to the side of the load owing the short-circuiting, it can be surely detected by the software in the microcomputer.
An object of the present invention is to enable the number of times of self-cutoff due to heat generation of a thermal MOSFET to be counted accurately.
In order to attain the above object, a method of protecting a semiconductor relay system including a thermal interrupting circuit incorporating type MOSFET, a microcomputer and a driving circuit, the MOSFET being on/off controlled by the microcomputer through the driving circuit so that the semiconductor relay system supplying a power source to a load, the method includes the steps of:
detecting a current supplied to the load;
controlling the driving circuit to cut off a voltage applied to a gate of the MOSFET so that supply of the power source to the load is stopped when the detected current is larger than a prescribed current value set at larger than a rated current value;
detecting a source voltage generated on a source side of the MOSFET while an instruction signal for turning on the MOSFET is outputted from the microcomputer;
counting a number of times which the detected source voltage is lower than a prescribed voltage value; and
turning off the semiconductor relay system for fear of reset when the counted number reaches a prescribed number of times.
In this configuration, the number of times of self-cutoff due to heat generated of a thermal MOSFET can be accurately counted.
In order to attain the above object, in a method of protecting a semiconductor relay system, the prescribed number of times is fewer than a number of times, defined by a characteristic of the MOSFET, that the MOSFET is broken of repetition of cutoff by a thermal self cut-off circuit incorporated in the MOSFET in state where a current limitation is imposed on a prescribed current value. In this configuration, the number of times of self-cutoff due to heat generated of a thermal MOSFET can be accurately counted.